Electric Current

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Electricity

K Warne

A VoltmeterVoltmeterAmmeterAmmeter

ResistorResistor

V2

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Electrical Circuits

A VoltmeterVoltmeterAmmeterAmmeter

ResistorResistor

The Ammeter measures The Ammeter measures the ……………flowing in the ……………flowing in the circuit. (…….. A)the circuit. (…….. A)

The Voltmeter Measures The Voltmeter Measures potential difference or potential difference or …………….. in volts. (V)…………….. in volts. (V)

The Resistance of the The Resistance of the Resistor is given in Resistor is given in …………… (Ω).…………… (Ω).

VV11 = V = V22 V2

Voltage across …………..= voltage in ……………Voltage across …………..= voltage in ……………





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Electric Current in a Conductor

Conventional current - __________________

Direct current - moves in _______________.

Alternating current - ___________________ continuously

Maintaining a current Conductor - ___________ _____________________ _____________________ -

SOURCE

+ -

+ -

+ +

+

+

+

+

+ ++ +

+

+

+

+

+ ++ +

+

+

+

+

+ ++ +

+

+

+

+

+ ++ +

+

+

+

+

+ +

e-

e-

e-

e- e- e-

e-

e- e-

e-

e-

e- e- e-

e-

e- e-

e- e-

e- e-+ - e-

e-

e-

<------- electrons

“_____________” ------>Conventional current is the movement of from + to - in a conductor.

Conventional current e-


Electric Current & Voltage

You can think of an electric CURRENT as a flow of charges – almost like people moving through a corridor.

The CURRENT is the NUMBER of charges (people ) passing any point in a second.

The VOLTAGE is how much ENERGY they have.

+ -

+ -

Conventional current e-

Vex

A

Resistance R

VCell

Emf 400V

I = 0A

Vcrt.= ….. v

Emf = …..cell

• Emf is the …………… amount of …………… that the cell can produce (per unit charge).

• Measured when the current in the circuit is ………..

EMF - Electro Motive Force

Open Circuit!!VVcellcell = EMF = EMF

As the number of resistors in As the number of resistors in

parallelparallel INCREASES INCREASES

•The total CURRENT flowing The total CURRENT flowing

..................................................

•The total RESISTANCE must The total RESISTANCE must therefore therefore

......................................................

•the BRIGHTNESS of the bulbs the BRIGHTNESS of the bulbs

..........................................................................

•The VOLTAGE across each bulb The VOLTAGE across each bulb

..................................................................................

•If the RESISTANCE is DOUBLED If the RESISTANCE is DOUBLED the CURRENT the CURRENT

................................................

Parallel circuit SummaryParallel circuit Summary

Images: crocodile clip screenshotsImages: crocodile clip screenshots


Parallel CircuitsParallel Circuits• AddingAdding resistors in resistors in parallel…parallel…

decreasesdecreases the total resistance the total resistance..11 11 11

RRtt RR11 RR22

11 11 11

RRtt … … ……

11 11 11 11

RRtt 2 2 22 2 2

RRt t = …./…. = ……. = …./…. = ……. Ω

= += +

R1 22 Ω

22 Ω

R1

R2

22 Ω

Total R = ….Total R = …. Ω

Total R = …Total R = … Ω

Total R = ……Total R = …… Ω

R1 22 Ω

22 ΩR2

R2 22 Ω

= += +

= += + + +





Ohm’s Law - PracticalAIM:

– Investigate the relationship between the …………………….. across a resistor and the ………………..flowing through it.

– Determine the …………………..of a resistor.

V

A

METHOD:

1. Set up the circuit as shown.

2. Using the rheostat vary the current in the circuit, obtain a range of readings for the potential difference across R for different currents.

3. RESULTS>>Resistance R

rheostat

Ohm’s Law - Analysis

I (A) V (V) V/I0.80 3.40 4.30.81 3.50 4.30.85 3.70 4.40.90 3.90 4.30.95 4.20 4.4

The ratio V/I produces a constant value - for any resistor

This is the Resistance of the resistor.

The Unit of measurement for resistance is the Ohm - symbol (Ω)

The SLOPE of the graph gives the RESISTANCE.

Current vs Voltage

3.00

3.20

3.40

3.60

3.80

4.00

4.20

4.40

0.75 0.80 0.85 0.90 0.95 1.00Current (A)

Volta

ge (V

)

RiseRiserunrun

Slope Slope = = Y/ Y/ XX = =


Current, voltage & resistance

We define the unit of resistance; one …….() is one volt per ampere.

R = V/I 1 = …V/…A

+ -

+ -

A V

The relationship between the …………… …………… through a resistor, the …………... drop across the resistor and the resistance of the resistor is expressed by the following equation:

RR

VV

Rx ..Rx ..

Calculate the voltage drop across a 2 resistor when a current of 1.5 A is flowing..


Worked Example4.14.1

Calculate ICalculate I


Effects of electric current An electric current that flows in a conductor has a number of

effects:

1. HEATING The friction caused by the current causes the conductor to heat up. The greater the current the more heat is generated.

2. MAGNETIC EFFECT - A magnetic field is generated around any conductor when an electric current flows through it.


Keith Warne

Hi -

This is a SAMPLE presentation only.

My FULL presentations, which contain loads more slides and other resources, are freely available on my resource sharing website:

Science Café.

www.sciencecafe.org.za

(paste into your browser if link above does not work)Have a look and enjoy!

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Education

Electric Current